Open MRI magnet assembly with paired planar driving coils having opposite current flows and a racetrack, nested, or substantially-planar multi-coil shape

ABSTRACT

The present invention provides a magnet assembly ( 30   a   , 30   b ) comprising a pair of drive coils ( 31   a   , 31   b   ; 32   a   , 32   b   ; 33   a   , 33   b ) symmetrically disposed with respect to a reference plane. The reference plane is located approximately equidistant between the drive coils. Each of the drive coils has a current flowing through it which is in the opposite sense to the current flowing through the other drive coil, such that a primary magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane. The assembly may comprise further compensating coils ( 44   a   , 45   a   , 46   a ) which are configured to generate a magnetic field to improve homogeneity of the primary magnetic field. The drive coils and the compensating coils are generally in the shape of a racetrack and are preferably superconducting coils.

The present invention relates to a magnet with improved access. Morespecifically, the present invention relates to a magnet for use in amagnetic resonance imaging (MRI) system which allows improved access tothe patient.

A magnetic field with a high degree of homogeneity is essential for thesuccessful application of a number of analytical techniques, inparticular MRI. These fields can be generated by a number of devices,such as coil magnets or permanent magnets, or a combination of the two.Ferromagnetic material is known to be used, in addition to the abovementioned magnets, to increase the field strength, improve fieldhomogeneity, and to limit stray magnetic fields.

Most known MRI magnets consist of an assembly of coaxial superconductingcoils. These coils are positioned in such a way that the required fieldstrength and homogeneity is achieved. The patient is positioned insidethe coils with the head-feet axis in line with the axis of the coils.FIG. 1 is an example of this type of MRI system, which is known in theart as the solenoid configuration. In FIG. 1 a cross section through thesolenoid type MRI system 10 is shown comprising a pair of drive coils 12a, 12 b, which function to generate the primary magnetic field {rightarrow over (B)} along the central line Z of the system. A plurality ofcompensating coils 13 a, 13 b, 13 c, 13 d are used to improve thehomogeneity of the primary magnetic field. The patient 15 is locatedalong the centre line Z of the system.

A disadvantage of the solenoid configuration shown in FIG. 1 is that thepatient has to be located inside a tube. This can cause the patientstress and make him feel trapped. Furthermore, access to the patientonce he is inside the tube is limited.

An object of the present invention is to provide a magnet whichgenerates a magnetic field suitable for use in an MRI system and whichallows improved access to the patient and which causes the patient lessstress.

According to the present invention there is provided a magnet assemblyfor use in an MRI system comprising a pair of drive coils, each of saidpair of drive coils being substantially the same size and shape andsymmetrically disposed with respect to a reference plane, the referenceplane being located between said pair of drive coils, each of said pairof drive coils being arranged such that a current flows there through,the current through the first drive coil of said pair being in anopposite sense to the current through the second drive coil of saidpair, such that a primary magnetic field is generated in a plane whichis parallel to and substantially coincident with the reference plane.

According to an aspect of the present invention, the magnet assemblyfurther comprises a pair of compensating coils, each of said pair ofcompensating coils being substantially the same size and shape andarranged such that a current flows there through, the current throughthe first compensating coil of said pair of compensating coils being inthe opposite sense to the current through the first drive coil, and thecurrent through the second compensating coil of said pair ofcompensating coils being in the opposite sense to the current throughthe second drive coil, such that a compensating magnetic field isgenerated in a plane which is parallel to and substantially coincidentwith the reference plane and which functions to compensate forinhomogeneity in the primary magnet field.

According to a further aspect, the first drive coil and the firstcompensating coil are disposed in a first plane, and the second drivecoil and the second compensating coil are disposed in a second plane.

According to yet a further aspect, the reference plane is approximatelyequidistant between the first and the second planes.

According to yet a further aspect, the pair of drive coils and the pairof compensating coils are superconducting coils.

According to yet a further aspect, the assembly comprises a plurality ofpairs of drive coils.

According to yet a further aspect, the assembly comprises a plurality ofpairs of compensating coils.

While the principle advantages and features of the invention have beendescribed above, a greater understanding and appreciation of theinvention may be obtained by referring to the drawings and detaileddescription of the preferred embodiment, presented by way of exampleonly, in which;

FIG. 2 is a cross section through a hypothetical two dimensional magnet,

FIG. 3 is a side view of a pair of improved access magnetic coilassemblies according to the present invention,

FIG. 4 is a plane view of an alternative embodiment of the improvedaccess magnetic coil assembly shown in FIG. 3, and

FIG. 5 is a MRI system incorporating the improved access magnetic coilassemblies shown in FIG. 3.

In FIG. 2 a cross section through a hypothetical two dimensional magnet20 is shown comprising conductors 21 a and 21 b, which are symmetricallydisposed with respect to a reference plane X-Z. The reference plane inlocated approximately equidistant between the conductors. Theoretically,conductors which are infinitely long would generate a primary magneticfield {right arrow over (B)} in a plane corresponding to the referenceplane and perpendicular to the head-foot axis of patient 15.

However, conductors with a finite length and disposed with the samevertical spacing Y between them can generate a magnetic field with thesame imaging quality as can theoretically be achieved with theinfinitely long conductors. This results in a MRI system withmuch-improved openness.

A good approximation of the ideal, infinitely long two-dimensionalmagnet shown in FIG. 2 can be achieved by truncating the length of theconductors and providing a return path for the currents that issubstantially away from the imaging region of the system.

In FIG. 3 a pair of improved access magnetic coil assemblies 30 a, 30 bare shown. Each assembly consists of three pairs of racetrack coils 31 a& 31 b, 32 a & 32 b, and 33 a & 33 b. The term racetrack coil is wellknown in the art and refers to coils that contain both arc and straightsections and are generally rectangular or oval shaped. However, as willbe appreciated by those skilled in the art, other shaped coils could beused. A patient 15 is located between the pair of magnet assemblies 30a, 30 b. Each racetrack coils 31 a, 32 a, 33 a in magnet assembly 30 aabove the patient is matched by a racetrack coil 31 b, 32 b, 33 b in themagnet assembly 30 b below the patient. Coils 31 a, 32 a, 33 a aresubstantially the same size and shape as coils 31 b, 32 b, 33 b. Coils31 a, 32 a, 33 a have a current running through them which is in theopposite sense to a current running through corresponding coils 31 b, 32b, 33 b. Thus for example, racetrack coil 31 a has a current running inthe direction indicated by arrow 34 a and the corresponding racetrackcoil 31 b has a current running in the opposite sense as indicated byarrow 34 b. Each corresponding pair of racetrack coils: 31 a & 31 b, 32a & 32 b, and 33 a & 33 b have substantially the same cross sectionalarea.

Each coil in an assembly has a current running through it which is in anopposite sense to the current running through the coil immediately nextto it. For example, in assembly 30 a, coil 31 a has a current runningthrough it in the opposite direction a current running through coil 33a. Coil 33 a has a current running through it in the opposite directiona current running through coil 32 a. The direction of current flow incoils 31 a, 33 a, 32 a is indicated by arrows 34 a, 36 a, 35 arespectively.

Similarly, in assembly 30 b, the currents running through coil 31 b isopposite to the current in coil 33 b, which is opposite to the currentin coil 32 b. The direction of current flow in coils 31 b, 33 b, 32 b isindicated by arrows 34 b, 36 b, 35 b respectively.

The coil assemblies 30 a, 30 b are symmetrically disposed with respectto a reference plane X-Z. The reference plane is located approximatelyequidistant between the coil assemblies. The coil assemblies generate aprimary magnetic field {right arrow over (B)} in the area between thecoil assemblies. The primary magnetic field {right arrow over (B)} is ina plane corresponding to the reference plane and perpendicular to thehead-foot axis of patient 15.

The coil assemblies also generate secondary magnet fields which are alsoparallel to and substantially coincident with the reference plane. Thesecondary fields function to compensate for inhomogeneity in the primarymagnetic field {right arrow over (B)}.

As will be appreciated, further pairs of coils may be included in themagnetic coil assemblies, which would further improve the homogeneity ofthe primary magnetic field.

Advantageously the configuration of coils in FIG. 3 generates a strongerhomogeneous magnetic field along the length of the patient 15 than ispossible with known solenoid type systems. This allows for imaging alonga greater length of the patient. For example, the entire spine of thepatient may be imaged in a single scan.

In FIG. 4 a single magnetic coil assembly 40 a is shown comprisingprimary racetrack coils 41 a, 42 a, 43 a and compensating racetrackcoils 44 a, 45 a, 46 a. The primary drive coils generate the primarymagnetic field and secondary fields for compensating for inhomogeneityin the primary field. The compensating coils generate further fieldswhich function to further improve the homogeneity of the primarymagnetic field. The area in which the primary magnetic field is suitablefor imaging is indicated by the sphere 48.

The magnetic fields generated by the X₁ arm of each racetrack coil areof primary importance to the imaging sphere 48 (shown by a dashed line).the return path of each racetrack coil, the X₂ arms, have an adverseeffect on the homogeneity of the primary magnetic filed and are thusdisposed at a great distance as possible from the imaging sphere. The Z₁and the Z₂ arms of each racetrack coil do not contribute significantlyto the primary magnetic field.

As will be appreciated by those skilled in the art, the distance atwhich the X₂ arms of the coils can be located from the X₁ arms willeffect the overall size of the MRI system. Thus it is desirable to keepthis distance to a minimum, while still maintaining a suitablyhomogenous primary magnet field. In an alternative embodiment, the X₂arms of each coil in the top coil assembly are closer to thecorresponding X₂ arms in the bottom coil assembly than the X₁ arms ofthe top and bottom coil assemblies. Advantageously, this further reducesthe size of the MRI system.

Furthermore, the diameter and shape of each racetrack coil will effectthe homogeneity of the primary magnetic field and thus can be varied inorder to provide the most optimum configuration.

In a preferred embodiment of the present invention, the racetrack coilsare superconducting coils and are housed within a cryostat.

FIG. 5 shows a MRI system 50 incorporating the racetrack magnetic coilassemblies shown in FIG. 3. The pair of magnet assemblies 30 a, 30 b aredisposed within cryostat structure 52. The patient 15 is positionedinside the MRI system 50. The primary magnetic field {right arrow over(B)} is parallel to, and substantially coincident with, the referenceplane X-Z and perpendicular to the head-foot axis of the patient.

Advantageously, the MRI system shown in FIG. 5 allows for the patient tobe more readily positioned within the system. Furthermore, access to thepatient is greatly increased allowing for the possibility of performingvarious medical procedures on the patient while he is still locatedwithin the MRI system.

As can be seen in FIG. 5, three sides of the MRI system are open. Thisresults in less stress to the patient due to the claustrophic nature ofknown solenoid type systems.

Furthermore, the arrangement shown in FIG. 5 allows for the possibilityof locating the pair of magnet assemblies 30 a, 30 b closer together inthe vertical direction Y. As is well known in the art, the closer thedrive coils are to each other, the more the overall size of each coiland the amount of conductor needed to make the coil can be reducedwithout adversely effecting the homogeneity of the primary magneticfield. Any reduction in overall coil size will advantageously reduce thesize and cost of the MRI system.

As will be appreciated by those skilled in the art, variousmodifications may be made to the embodiment hereinbefore describedwithout departing from the scope of the present invention.

1. An MRI system comprising: a magnet assembly configured for generatinga stationary magnetic field, the magnet assembly comprising: first andsecond substantially planar coil assemblies, each coil assemblycomprising at least two drive coils; each drive coil in the first coilassembly, forming a respective pair of drive coils with a correspondingdrive coil in the second coil assembly, each drive coil of each pair ofdrive coils being substantially planar and of a substantially same sizeand shape as the other of the respective pair, said drive coils in eachpair of drive coils being substantially parallel to one another andsubstantially parallel to an intermediate reference plane, and beingdisposed approximately equidistant from, and symmetrically with respectto, the intermediate reference plane; wherein; each drive coil of eachpair of drive coils is arranged in order to enable a current to flowtherethrough, the current through a first drive coil of each respectivepair being in an opposite sense to the current through a second drivecoil of said respective pair, whereby, in each respective pair of drivecoils, a first section of the first drive coil and a corresponding firstsection of the second drive coil generate a primary magnetic fieldextending within the intermediate reference plane; and in eachrespective pair of drive coils, a second section of the first drivecoil, opposite to the first section of the first drive coil, and acorresponding second section of the second drive coil, opposite to thefirst section of the second drive coil constitute respective returnpaths of the first and second drive coils in said respective pair, andare disposed at a sufficient distance from the first sections of thedrive coils that magnetic fields generated by the second sections do notsubstantially adversely affect the homogeneity of the primary magneticfield generated by the first sections.
 2. The MRI system according toclaim 1, wherein: the drive coils in the first and second coilassemblies are located in a single plane; each respective drive coil ofthe first coil assembly is arranged in order to carry a current in arespective first sense; each respective drive coil of the secondassembly is arranged in order to carry a current in a respective secondsense, opposite to the respective first sense; and each respective drivecoil in each coil assembly is arranged in order to carry its current ina respective sense opposite to that of the current in each adjacentdrive coil in the respective coil assembly.
 3. An MRI apparatus asclaimed in claim 2, wherein the first and second coil assembliesrespectively further comprise first and second coils of a pair ofcompensating coils, each of said coils of said pair of compensatingcoils being substantially parallel to one another and substantiallyparallel to an intermediate reference plane, and being of asubstantially same size and shape and disposed equidistant from, andsymmetrically with respect to, the intermediate reference plane, andeach compensating coil being arranged to enable a current to flowtherethrough, the current through a first compensating coil of said pairbeing in an opposite sense to the current through a second compensatingcoil of said pair; whereby a first section of the first compensatingcoil and a corresponding first section of the second compensating coilgenerate a compensating magnetic field within the intermediate referenceplane; and wherein a second section of the first compensating coil,opposite to the first section of the first compensating coil, and acorresponding second section of the second compensating coil, oppositeto the first section of the second compensating coil, constituterespective return paths of the first and second compensating coils,respectively, and are disposed at a sufficient distance from the firstsections of the compensating coils that magnetic fields generated by thesecond sections of the compensating coils do not substantially adverselyaffect the homogeneity of the primary magnetic field generated by thefirst sections of the drive coils.
 4. An MRI apparatus as claimed inclaim 3, wherein each drive coil of said pair of drive coils areracetrack shaped.
 5. An MRI apparatus as claimed in claim 4, whereineach compensating coil of said pair of compensating coils are racetrackshaped.
 6. An MRI apparatus as claimed in claim 5, wherein said drivecoils and said compensating coils are superconducting coils.
 7. An MRIapparatus as claimed in claim 6, wherein the magnet assembly comprises aplurality of pairs of compensating coils.
 8. An MRI system comprising amagnet assembly configured for generating a stationary magnetic field,the magnet assembly comprising first and second substantially planar andmirror symmetrically disposed coil assemblies, wherein: each coilassembly comprises a plurality of drive coils; each drive coil in thefirst coil assembly forms a respective pair of drive coils with acorresponding drive coil in the second coil assembly; each drive coil ofeach pair of drive coils is substantially planar and of a substantiallysame size and shape as the other of the respective pair; the drive coilsin each pair of drive coils are substantially parallel to one anotherand substantially parallel to an intermediate reference plane, and aredisposed approximately equidistant from, and symmetrically with respectto, the intermediate reference plane; the drive coils within eachsubstantially planar coil assembly are nested within one another in asingle plane of said coil assembly and have differing length and widthdimensions; each drive coil of each pair of drive coils is arranged inorder to enable a current to flow therethrough; the current through afirst drive coil of each respective pair is in an opposite sense to thecurrent through a second drive coil of said respective pair, whereby, ineach respective pair of drive coils, a first section of the first drivecoil and a corresponding first section of the second drive coil generatea primary magnetic field extending within the intermediate referenceplane; said first sections of each of said plurality of drive coils ofeach coil assembly are disposed in parallel, in substantially closeproximity to each other; in each respective pair of drive coils, asecond section of the first drive coil, opposite to the first section ofthe first drive coil, and a corresponding second section of the seconddrive coil, opposite to the first section of the second drive coilconstitute respective return paths of the first and second drive coilsin said respective pair; and said second sections of each of saidplurality of drive coils are disposed at a distance from the firstsections of the drive coils, which distance is sufficient so thatmagnetic fields generated by said return paths do not substantiallyadversely affect the homogeneity of the primary magnetic field generatedby the first sections.
 9. The MRI system according to claim 8, wherein:each respective drive coil of the first coil assembly is arranged inorder to carry a current in a respective first sense; each respectivedrive coil of the second assembly is arranged in order to carry acurrent in a respective second sense, opposite to the respective firstsense; and each respective drive coil in each coil assembly is arrangedin order to carry its current in a respective sense opposite to that ofthe current in each adjacent drive coil in the respective coil assembly.10. An MRI apparatus as claimed in claim 9, wherein the first and secondcoil assemblies respectively further comprise first and second coils ofa pair of compensating coils, each compensating coil of said pair ofcompensating coils being substantially parallel to one another andsubstantially parallel to an intermediate reference plane, and being ofa substantially same size and shape and disposed equidistant from, andsymmetrically with respect to, the intermediate reference plane, andeach arranged to enable a current to flow therethrough, the currentthrough a first compensating coil of said pair being in an oppositesense to the current through a second compensating coil of said pair;whereby a first section of the first compensating coil and acorresponding first section of the second compensating coil generate acompensating magnetic field extending within the intermediate referenceplane; and wherein a second section of the first compensating coil,opposite to the first section of the first compensating coil, and acorresponding second section of the second compensating coil, oppositeto the first section of the second compensating coil, constituterespective return paths of the first and second compensating coils,respectively, and are disposed at a sufficient distance from the firstsections of the compensating coils that magnetic fields generated by thesecond sections of the compensating coils do not substantially adverselyaffect the homogeneity of the primary magnetic field generated by thefirst sections of the drive coils.
 11. An MRI apparatus as claimed inclaim 10, wherein each drive coil of said pair of drive coils areracetrack shaped.
 12. An MRI apparatus as claimed in claim 11, whereineach compensating coil of said pair of compensating coils are racetrackshaped.
 13. An MRI apparatus as claimed in claim 12, wherein said drivecoils and said compensating coils are superconducting coils.
 14. An MRIapparatus as claimed in claim 13, wherein the magnet assembly comprisesa plurality of pairs of said compensating coils.